The present invention relates to a thermal transfer material for use in a method for forming a color image, particularly a multi-color or full-color image by melt-transferring heat-meltable inks onto a receptor having a multiplicity of micropores in the surface layer thereof.
Heretofore there has been proposed a method for forming a multi-color image on a receptor having a multiplicity of micropores in the surface layer thereof wherein a yellow heat-meltable ink layer, a magenta heat-meltable ink layer and a cyan heat-meltable ink layer are selectively melt-transferred in succession onto the receptor to enter each ink in a molten state into the micropores, thereby forming a multi-color image on the basis of subtractive color mixture (Institute of Television Engineers of Japan (ITE) Technical Report, Vol. 17, No. 27, pages 19 to 24 (May, 1993).
The color image formation method is explained by referring to FIGS. 1 and 2. In FIG. 1, numeral 1 denotes a thermal transfer material wherein heat-meltable ink layers 3 for respective colors are provided on a foundation 2. Numeral 4 denotes a receptor wherein a multiplicity of micropores 5 are formed in the surface layer thereof (hereinafter referred to as "porous surface receptor" in some cases). The diameter and depth of the micropores 5 are on the order of micrometers. In the porous surface receptor 4 shown in FIG. 1, the micropores 5 are pictured regularly but actual micropores are irregular.
The thermal transfer material 1 is superimposed onto the receptor 4. The resulting assembly is heated by means of a thermal head T (in FIG. 1, only one heating element is shown) with being pressed against a platen P, whereby the ink in a heated portion is melted and the molten ink is entered into micropores 5 mainly by capillary action. When the thermal transfer material 1 is separated from the receptor 4, there is obtained the receptor 4 having a color image wherein the ink 6 is contained in the micropores 5 in a portion of the receptor 4 which corresponds to the activated heating elements of the thermal head T, as shown in FIG. 2.
The development of a color, for example, red, on the basis of subtractive color mixture can be achieved by first entering a yellow ink 6Y into micropores 5 and then entering a magenta ink 6M into the micropores 5, thereby superimposing both inks in the respective micropores 5, as shown in FIG. 3. Similarly, green is obtained by a combination of yellow ink and cyan ink; blue is obtained by a combination of magenta ink and cyan ink; and black is obtained by a combination of yellow ink, magenta ink and cyan ink.
In the color image formation method, the density of each color is determined by the amount of the ink for the color contained in the micropores of the receptor. Therefore the method has an advantage that the representation of gradation is possible in every picture element by controlling the amount of each ink heated in transfer.
However, research has not been fully made on the thermal transfer material for use in the aforesaid color image formation method. The present inventors research has revealed various problems including the difficulty in entering a predetermined amount of an ink into the micropores.
A serious problem is that as shown in FIG. 5, there occurs a phenomenon that the ink transferred onto the receptor is not sure to get into the micropores 5, hence, a portion of the ink remains in the form of a layer on the surface of the receptor 4 (hereinafter referred to as "excess transfer"). When such an excess transfer, which means that a predetermined amount of the ink does not get into the micropores occurs, a good gradation and a desired subtractive color mixture are not achieved, resulting in a poor color reproducibility, and the ink is not transferred in the same area as that of the heating element, resulting in a decrease in resolution.
In view of the above, an object of the present invention is to provide a thermal transfer material useful for the foregoing color image formation method.
Another object of the present invention is to provide a thermal transfer material capable of forming a multi-color or full-color image excellent in gradation, color reproducibility and resolution.
These and other objects of the present invention will become apparent from the description hereinafter.